The astrometry is determined either by you supplying explicit values for certain projection parameters, or by you providing the sky and corresponding image co-ordinates for a set of positions (see Parameter POSITIONS). In the latter case, the projection parameters are determined automatically by searching through parameter space in order to minimise the sum of the squared residuals between the supplied pixel co-ordinates and the transformed sky co-ordinates. You may force particular projection parameters to take certain values by assigning an explicit value to the corresponding application parameter listed below. The individual residuals at each position can be written out to a logfile so that you can identify any aberrant points. The RMS residual (in pixels) implied by the best-fitting parameters is displayed.
[pixelsize] [orient] [tilt] [logfile]
"Equatorial"(FK4 and FK5), and
"Galactic"(IAU 1958). Ecliptic and equatorial co-ordinates are referred to the mean equinox of a given epoch. This epoch is specified by appending it to the system name, in parentheses, for example,
"Equatorial(1994.5)". The epoch may be preceded by a single character,
"J", indicating that the epoch is Besselian or Julian respectively. If this letter is missing, a Besselian epoch is assumed if the epoch is less than 1984.0, and a Julian epoch is assumed otherwise.
TRUEvalue is supplied, then the WCS information will be stored in the form of an IRAS90 astrometry structure. This is the form used by the IRAS90 package (see SUN/163). In this case, any existing IRAS90 astrometry structure will be over-written. See the “Notes” section below for warnings about using this form.
FALSE value is supplied, then the WCS information will be stored in the form of a
standard NDF WCS component which will be recognized, used and updated correctly by most
other Starlink software.
If a null value (
!) is supplied, then a
TRUE value will be used if the supplied NDF
already has an IRAS90 extension. Otherwise a
FALSE value will be used.
"Equatorial", LAT is the declination. See SUN/163, Section 4.7.2 for full details of the allowed syntax for specifying this position. For convenience here are some examples how you may specify the declination -45 degrees, 12 arcminutes:
"-45 12 00",
"-0.78888r". The last of these is a radians value. A null value causes the latitude of the reference point to be estimated automatically from the data supplied for Parameter POSITIONS.
"Equatorial", LON is the right ascension. See SUN/163, Section 4.7.2 for full details of the allowed syntax for specifying this position. For convenience here are some examples how you may specify the right ascension 11 hours, 34 minutes, and 56.2 seconds:
"11 34 56.2",
"11h 34m 56.2s",
"113456.2". See Parameter LAT for examples of specifying a non-equatorial longitude. A null value causes the longitude of the reference point to be estimated automatically from the data supplied for Parameter POSITIONS.
"Pixel". Remember that the centre of a pixel at indices i,j is . A null value causes the pixel co-ordinates of the reference point to be estimated automatically from the data supplied for Parameter POSITIONS.
"2.618E-6r". A null value causes the pixel dimensions to be estimated automatically from the data supplied for Parameter POSITIONS.
"Aitoff"–- Aitoff equal-area,
"Gnomonic"–- Gnomonic (i.e. tangent plane),
"Lambert"–- Lambert normal equivalent cylindrical,
The following synonyms are also recognised:
"All_sky" –- Aitoff,
"Tangent_plane" –- Gnomonic.
See SUN/163 for descriptions of these projections. A null value causes the projection
to be determined automatically from the data supplied for Parameter POSITIONS.
"Pixel"this requests that pixel co-ordinates for the reference point be obtained through Parameter PIXELREF. The other options are locations specified by two characters, the first corresponding to the vertical position and the second the horizontal. For the vertical, valid positions are
C(entre); and for the horizontal the options are
C(entre). Thus REFCODE=
"CC"means the reference position is at the centre of the NDF image, and
"BL"specifies that the reference position is at the centre of the bottom-left pixel in the image. A null value causes the pixel co-ordinates of the reference point to be estimated automatically from the data supplied for Parameter POSITIONS.
stars.lis. This file contains a line for each position, each line containing an ecliptic longitude and latitude, followed by a pair of image co-ordinates. These values should be separated by commas. The ecliptic co-ordinates were determined at Julian epoch 1994.0, and are referred to the mean equinox at Julian epoch 1994.0. The determined parameter values together with the residual at each position are logged to file
The Gaia image display tool (SUN/214) provides various interactive tools for storing new WCS information within an NDF.
This application was written to supply the limited range of WCS functions required by the IRAS90 package. For instance, it does not support the complete range or projections or sky co-ordinate systems which may be represented by the more general NDF WCS component.
If WCS information is stored in the form of an IRAS90 astrometry structure (see
Parameter IRAS90), it will in general be invalidated by any subsequent Kappa commands
which modify the transformation between sky and pixel co-ordinates. For instance, if
the image is moved using SLIDE (for example), then the IRAS90 astrometry structure will
no longer correctly describe the sky co-ordinates associated with each pixel. For this
reason (amongst others) it is better to set Parameter IRAS90 to